The present invention relates to an electrode comprising a conductive carbon material especially for high performance applications, such as used in supercapacitors, and to methods of preparing such an electrode.
Supercapacitors have attracted great interest in the energy storage field because they complement batteries with respect to energy and power densities as is described by A. S. Arico, P. Bruce, B. Scrosati, J.-M. Tarascon, W. Van Schalkwijk, in Nature Mater., 2005, 4, 366, and by R. Röz, M. Carlen, in Electrochim. Acta, 2000, 45, 2483. Supercapacitors have potential applications in hybrid transportation systems whenever high power densities are needed, e.g. in providing high power during acceleration, deceleration or braking. Based on the charge-storage mechanism, supercapacitors can be divided into two categories as described by B. E. Conway, in Electrochemical Supercapacitors, Scientific Fundamentals and Technological Applications, Kluwer Academic/Plenum Publishers, New York, 1997, and by A. Burke, in J. Power Sources, 2000, 91, 37. One of these categories is the electrical double layer capacitor (EDLC), where the capacitance arises from the charge separation at an electrode/electrolyte interface, e.g. at a carbon electrode. EDLC capacitors are for example described by E. Frackowiak, F. Béguin, in Carbon, 2001, 39, 937, by A. G. Pandolfo, A. F. Hollenkamp, in J. Power Sources, 2006, 157, 11, and by J. Chmiola, G. Yushin, Y. Gogoti, C. Portet, P. Simon, P. L. Taberna, in Science, 2006, 313, 1760. The other category is the redox capacitor, where the capacitance comes from Faradaic reactions at the electrode/electrolyte surface of e.g. transitional metal oxides or of electroactive polymers. Redox capacitors using transition metal oxides are described by K. H. Chang, Y. T. Wu, C. C. Hu, in Recent advances in supercapacitors. ed. V. Gupta, Transworld Research Network, Kerala, India, 2006, P30, by C. C. Hu, T. W. Tsou, in Electrochem. Commun., 2002, 4, 105, by M. Wu, G. A. Snook, G. Z. Chen, D. J. Fray, Electrochem. Commun., 2004, 6, 499, and by D. Choi, G. E. Blomgren, P. N. Kumta, in Adv. Mater., 2006, 18, 1178. Redox capacitors using electroactive polymers are described by A. Rudge, J. Davey, I. Raistrick, S. Gottesfeld, J. P. Ferraris, in J. Power Sources, 1994, 39, 273, by A. Rudge, I. Raistrick, S. Gottesfeld, J. P. Ferraris, in Electrochim. Acta, 1994, 47, 89, and by C. Arbizzani, M. C. Gallazzi, M. Mastragostino, M. Rossi, F. Soavi in Electrochem. Commun., 2001, 3, 16.
In EDLCs, the capacitance is proportional to the surface area of the electrode/electrolyte interface, so the performance is limited by the surface area of the electrode materials. Although theoretically surface areas as high as 2600 m2 g−1 can be achieved for nanoporous carbon, and thus specific capacitances of 400 F g−1 can be obtained, this is not a realistic option from a cost point of view. Generally, for an activated carbon with a specific surface area of 1000 m2 g−1, the specific capacitance is 150 F g−1, see C. Vix-Guterl, E. Frackowiak, K. Jurewicz, M. Friebe, J. Parmentier, F. Béguin, in Carbon, 2005, 43, 1293. In addition to the surface area, the composition in carbon materials is important as the introduction of heteroatoms in the carbon network gives rise to pseudocapacitive storage as described by F. Béguin, K. Szostak, G. Lota, E. Frackowiak, in Adv. Mater., 2005, 17, 2380, by E. Raymundo-Piñero, F. Leroux, F. Béguin, in Adv. Mater., 2006, 18, 1877, and by W. Li, D. Chen, Z. Li, Y. Shi, Y. Wan, J. Huang, J. Yang, D. Zhao, Z. Jiang, Electrochem. Commun., 2007, 9, 569 (e.g. a high specific capacitance of 200 F g−1 can be obtained by using carbon (SBET=273 m2 g−1) doped with a high amount of oxygen).
Compared with EDLCs, redox capacitors exhibit higher specific capacitances; e.g. record values of ˜982 F g−1 and ˜385 F g−1 have been respectively obtained for hydrous RuO2 (see O. Barbieri, M. Hahn, A. Foelske, R. Kötz, J. in Electrochem. Soc., 2006, 153, A2049), and anhydrous nanoporous RuO2 (see Y.-S. Hu, Y.-G. Guo, W. Sigle, S. Hore, P. Balaya, J. Maier, in Nature Mater., 2006, 5, 713). However, the high cost of noble metal materials inhibits their commercial application. Relatively low cost materials such as MnOx can also be used as electrode materials, but the specific capacitances (˜400 F g−1) still need to be enhanced. This is described by T. Shinomiya, V. Gupta, N. Miura, in Electrochim. Acta, 2006, 51, 4412. Electroactive polymers have advantageous properties with respect to low cost, high conductivity, high doping/dedoping rate during charge/discharge processes as well as facile synthesis through chemical and electrochemical methods, such as described by A. Malinauskas, J. Malinauskiene, A. Ramanavicius, in Nanotechnology, 2005, 16, R51. However, they exhibit the disadvantage of a low cycle life because swelling and shrinkage may occur during doping/dedoping processes, thus leading to mechanical degradation of the electrodes and fading of the electrochemical performance as described by E. Frackowiak, F. Béguin, in Recent advances in supercapacitors. ed. V. Gupta, Transworld Research Network, Kerala, India, 2006, P79.
Over the years much attention has been paid to the synthesis of electrode materials with highly electroactive regions by controlling the microstructure (i.e. grain size, thickness, specific surface area and pore characters). Reference is made in this connection to M. D. Ingram, H. Staesche, K. S. Ryder, Solid State Ionics, 2004, 169, 51, and to L.-Z. Fan, J. Maier, in Electrochem. Commun., 2006, 8, 937. Nanometer-sized electroactive materials with high porosities in contact with liquid electrolytes can exhibit enhanced electrode/electrolyte interface areas, providing highly electroactive regions and decreased diffusion lengths within active materials as described by Prof. J. Maier in Nature Mater., 2005, 4, 805. The use of carbon nanotubes with exceptional conducting and mechanical properties as a support for active materials can not only increase the specific capacitance of active materials, but also relieve the cycle degradation problems caused by mechanical problems. This has been described by many authors, such as M. Hughes, M. S. P. Shaffer, N. C. Renouf, C. Singh, G. Z. Chen, D. J. Fray, A. H. Windle, in Adv. Mater., 2002, 14, 382, by M. Hughes, G. Z. Chen, M. S. P. Shaffer, D. J. Fray, A. H. Windle, in Chem. Mater., 2002, 14, 1610, by K. Jurewicz, S. Delpeux, V. Bertagna, F. Béguin, E. Frackowiak, in Chem. Phys. Lett., 2001, 347, 36, by V. Khomenko, E. Frackowiak, F. Béguin, in Electrochim. Acta, 2005, 50, 2499, by V. Gupta, N. Miura, in J. Power Sources, 2006, 157, 616, by E. Frackowiak, V. Khomenko, K. Jurewicz, K. Lota, F. Béguin, in J. Power Sources, 2006, 153, 413, and by V. Gupta, N. Miura, in Electrochim. Acta, 2006, 52, 1721.
Up to now the highest specific capacitance reported for a polymer material application (PANI— polyaniline in a PANI/carbon composite) is a capacitance per mass of PANI of 1221 F g−1. There whisker-like PANI was grown on mesoporous carbon by a chemical polymerization method as described by Y.-G. Wang, H.-Q. Li, Y.-Y. Xia, in Adv. Mater., 2006, 18, 2619.